The fasteners that hold together multi-layer structures, particularly those structures that are subject to significant dynamic forces and/or pressure, such as aircraft and other vehicle bodies, bridges, buildings, and others, must be properly secured in order to ensure that the structure will perform as intended over its lifetime. In order to properly secure a fastener in a multi-layer structure, the fastener hole must not have any sharp edges, i.e., burrs, the structure must be free of debris between the layers, and any sealant applied between the layers in order to make the structure air and/or water tight must be sufficiently squeezed out. When excess sealant is present between the layers, the distance between the layers is increased and/or uneven, which may be referred to as a “gasket” condition.
Thus, if burrs, debris and/or excessive sealant are present, then the layers cannot be properly fastened and the layers may suffer corrosion, cracking and/or premature fatigue failure, which generally renders the structure ineffective for its intended purpose and, therefore, subject to the expense of repair or replacement.
Thus, ensuring that a burr-less hole is drilled, that there is no debris between the layers, and that sealant is properly applied between the layers is an integral part of clamping multi-layer structures together. In the aerospace industry, for example, a significant amount of time and labor are expended to ensure that the holes through the various layers of the aircraft structure are appropriately drilled, cleaned, sealed and fastened. Initially, the layers of materials that form the structure are loosely assembled without sealant, and drill templates are aligned and attached to the structure in the areas to be drilled. A drill operator, guided by the drill template, then drills holes through the layers of materials typically using a manual drill motor.
As the hole is being drilled through the layers, the drill bit tip pushes with the full feed force applied to the drill motor. This can cause a gap to develop between a layer currently being drilled and an underlying layer, particularly when the layers are a stack-up of thin material. The gap between the layers may cause burrs about the hole and debris may gather between the layers. Thus, once the holes are drilled, the layers must be disassembled, the burrs must be removed from about the holes, and the debris must be cleaned from the surfaces of the layers, all of which is a time-consuming and labor intensive process.
Sealant is then applied to the layers prior to re-assembling the layers. In order to ensure the layers are properly sealed to provide an air and water-tight seal, a generous amount of sealant is applied to the layers. Clamps that extend through the holes must be placed through each hole of the reassembled layers in order to squeeze out the sealant to prevent excessive “gasket” between the layers before the sealant dries. The extra sealant squeezes out around the clamps and must be cleaned from the structure and the clamps during clamping and/or after the clamps are removed.
Once the sealant is cured, the clamps are removed and the holes may be countersunk. In order to countersink a hole, a countersink drill bit and microstop countersink cage are attached to the drill motor and the operator revisits each hole to drill the countersink. The holes are then inspected to ensure they were properly countersunk. The holes may be inspected by checking the countersink of each hole with a measuring tool, or by placing a fastener in the hole to check if it fits properly within the hole and countersink. If the holes are satisfactory, then fasteners may be installed and fastened with nuts or swage lock collars. Overall, this process is expensive, laborious, and time-consuming.
In addition, the integrity of the resulting holes depends upon the completion of many manual processes, which creates a risk that certain steps may be performed inadequately or completely overlooked.
For structures that have a flat outer surface, a positive, power feed drill motor with a collet mandrel is capable of drilling, reaming and countersinking holes in multi-layer structures in one step. Positive power feed drill motors with collet mandrels are commercially available. To utilize these drill motors, a hole is initially drilled through the layers of material by conventional means, such as by using a conventional drill motor to drill a hole through the layers, then de-burring the hole and cleaning the debris from between the layers.
Once the structure is reassembled, the collet is extended through the hole and secured with the mandrel on the side of the structure that is opposite the drill motor. The collet mandrel is typically attached to a platform of the drill motor such that the platform sets on the surface of the structure. The platform also generally has an opening that is aligned with the structure to be drilled, and the drilling tool of the drill motor may extend through the platform opening to drill through the structure. Thus, the collet mandrel securely holds the layers of the structure together such that holes may be drilled and countersunk within a predetermined distance from the collet mandrel in one step without the risk of debris accumulating between the layers.
Once all of the desired holes have been drilled within the predetermined distance from the collet mandrel, the collet mandrel is removed from the hole and moved to one of the holes recently drilled or a new hole must be drilled for the collet mandrel, prior to repeating the process described above to form additional holes.
Holes are preferably drilled at a predefined orientation, such as a normal orientation, relative to the surface of the structure. As long as the surface of the structure is flat, the drilling tool of the drill motor will typically be normal to the structure to be drilled because the platform lies flat on the surface and the platform is attached normal to the drilling tool. Thus, so long as the platform is normal to the drilling tool, then the surface of the structure will also be normal to the drilling tool. If, however, the surface is contoured (concave, convex, or otherwise extending in three dimensions), the drilling tool may not be normal to the area of the structure to be drilled because the platform cannot lie flat on the surface of the structure.
The drill motor with the collet mandrel described above may include an adjustable footpad to adjust the position of the platform, and therefore the drilling tool until the drilling tool is normal to the area of the structure to be drilled. The adjustable footpad is typically attached to the drill motor platform opposite the side of the platform with the opening, and is adjustable to various positions based upon the radius of curvature of the surface and the distance from the footpad to the drilling tool. This design is suitable for simple contours with only one radius of curvature between the footpad and the drilling tool, i.e., for surfaces that curve in only one dimension, because the adjustable footpad maintains contact with the contoured surface across the full width of the pad.
If, however, more than one radius of curvature is present between the foot pad and the drilling tool, such as in complexly curved surfaces exhibiting curvature in two or more dimensions, the pad cannot maintain contact with the surface across the width of the pad because the curvature of the surface may be different at one side of the foot pad than it is at the other side of the foot pad. Thus, the conventional adjustable footpad is not capable of ensuring that the drilling tool is normal or in any other predefined relationship to the surface of the structure to be drilled when the surface of the structure is complexly curved.
As such, to drill a hole in a multi-layer structure having a complex curvature, the holes must be drilled in the conventional manner of drilling, de-burring, cleaning the layers, then reassembling and sealing the layers before countersinking the holes, which is a time-consuming and labor-intensive process, as discussed above.
A modified drilling apparatus 10 that contains adjustable mechanisms that allow for the positioning of the platform to be modified is shown in FIG. 1. The adjustment mechanism shown in the embodiment of FIG. 1 has a template foot pad housing 12 and at least two adjustable members 14. The drilling apparatus 10, therefore, may be placed in a desired orientation with respect to the surface of the structure 18 by appropriately adjusting each of the adjustable members 14. For example, the drilling apparatus 10 may be oriented such that the longitudinal axis of the drilling tool 20 is normal to the point on the surface of the structure 18 to be drilled, if it is desired that the resulting drilled hole be normal to the surface. A clamping element 22 is attached to the platform 24 of the drilling apparatus 10, and extends through an existing hole in the layers of the structure 18. The drilling apparatus 10 includes a bushing 26, i.e. a template boss, that at least partially surrounds the drilling tool 20. The bushing 26 may therefore be aligned with, such as by being at least partially positioned within, another hole in the drill template in order to, therefore, align the drilling tool 20 with the area of the structure 18 to be drilled. However, this design requires regular interaction with the adjustable members 14 and manually orientating the platform 24 and drilling tool 20 to achieve a normal orientation between the drilling tool 20 and the structure 18.